This is the Chain Viper, named after one of the deadliest and most aggressive vipers in the world. They are the elite of the Cobra Viper corps, tasked with special operations such as long-range reconnaissance and surveillance, and elimination of high-value targets. The idea of this custom is that it is the type of Viper that Mercer was before he defected to the Joes, which is why the body is a mashup of a Viper and Mercer v2's outfit.